The present invention relates to a light emitting semiconductor device, such as a semiconductor laser having light wave guide built therein.
Recently, semiconductor devices have been developed which consists of a light wave guide formed by burying an active layer in a groove in a semiconductor substrate and enclosing the active layer within low-refractive indexed semiconductor materials, e.g., InP or an AlGaAs.
These semiconductor systems exhibit a zinc blend type crystal structure and expose conventionally a (111) A facet by etching a (100) plane of the crystal.
Zh. L. Alferov, et al., "AlGaAs Heterostructure with Confined Current Flow" in Sov. Phys. Tech. Phys. 22 (8) 1977, pp. 1032 1036 (August, 1977), recites that (111) A facets are exposed by etching the crystal with various etchants. However, Alferov, et al. does not teach to expose a (111) B facet.
W. Suzaki, et al. to Mitsubishi Denki K.K. (Mitsubishi Electric Corporation), "Japanese Patent Application Laid-Open No. 56-158496", discloses that an InGaAsP/InP laser is produced by furying an InGaAsp active layer in InP by a two-step liquid phase epitaxial technique and that a dovetail-shaped groove is etched along the &lt;011&gt; direction and a V-shaped groove is etched orthogonal to the dovetail groove, i.e., parallel to the &lt;011&gt; direction.
These grooves, either dovetail-shaped or V-shaped, have inclined side planes of a (111) A facet. The shapes and directions of these grooves are illustrated in FIG. 1. However, the grooves 1 and 2 having a (111) A facet have the following disadvantages. A dovetail-shaped groove 1 has a curved active layer expitaxially grown along the curvature of the bottom of the groove, which leads to unstable transverse mode oscillation. In addition, the side edges of the groove 1 tend to be etched off by melting. A V-shaped groove often suffers from poor epitaxial growth, because it is not always filled up by the grown layers.
The present inventors conducted experiments to determine the percentage of successfully grown InGaAsP active layers in the transverse section of InP V-shaped grooves having (111) A facets. The results are shown in Table 1, which shows the relationship between the percentage of successfully grown active layers, the cooling rate, and supercooling. As shown in the table, the yield is generally low. In addition, further defective portions remained in the longitudinal direction of the grooves. Therefore, it is impossible to use the resultant products as lasers.
TABLE 1 ______________________________________ Percentage of Successful Growth (unit: %) Cooling rate (.degree.C./min) Supercooling (.degree.C.) 0.05 0.1 0.2 0.3 0.7 ______________________________________ 7 -- 85 83 38 20 9 -- 85 -- -- -- 10 90 80 82 30 -- 11 62 59 53 -- -- 14 60 52 -- 30 0 ______________________________________
Robert D. Burnham, et al. to Xerox Corporation, "Japanese Patent Laid-Open No. 52-3392", discloses an AlGaAs/GaAs light emitting semiconductor device in which the AlGaAs active layer within a groove is connected to AlGaAs layers grown outside of the groove. The current which flows through the layers outside of the groove does not pass through the active layer. FIG. 5 shows the relationship between light output and current in InGaAsP/InP light emitting semiconductor devices obtained by our experiments. The upper width of the grooves were 3.6 .mu.m and the measurement was performed at a temperature of 50.degree. C. Curve A was obtained using a device in which the InGaAsP layer 15 in the groove 13 was separated from those outside of the groove 13, as shown in FIG. 2; curve B was obtained using a device in which the InGaAsP layer 15 continuously covers the n-InP layer 14, as shown in FIG. 4. The light output of curve B is relatively low in relation to the current. Thus, it is clear that the side edges of an active layer should not contact an InGaAsP layer outside of the groove.